OvmfPkg/OvmfXen: consume PciHostBridgeLibScan

OvmfPkg/OvmfXen: consume PciHostBridgeLibScan

Switch the OvmfXen platform from the "OvmfPkg/PciHostBridgeLib" instance
to the "OvmfPkg/PciHostBridgeLibScan" instance. Currently this is a no-op
functionally; we'll customize the "PciHostBridgeLibScan" instance later.

Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Julien Grall <julien@xen.org>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2122
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20210526201446.12554-32-lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>